Conventionally, in measuring a wavelength or spectrum of light, a dispersion element having angular dispersion properties for a wavelength of light (for example, a diffraction grating, a prism, or an etalon), an interferometer, or the like has been used (for example, see Patent Literature 1).
In such measurement, a measurable range of a light band and an accuracy of the measurement have a trade-off relationship. Generally, measurement applicable for a wide light band has an accuracy of approximately several nanometers, while measurement applicable for a narrow light band has an accuracy of approximately several picometers.